Remove the orphaned onnxruntime Dockerfiles

#11803 dropped onnx and flax from the docker build matrix in
build_docker_images.yml. #12151 finished the flax half by deleting
docker/diffusers-flax-*, but the two onnxruntime directories were left
behind, so docker/ has listed two images for thirteen months that
neither job in the workflow builds. Their :latest tags on Docker Hub
have been frozen since 2025-06-27 as a result.

Delete docker/diffusers-onnxruntime-cpu and
docker/diffusers-onnxruntime-cuda, mirroring what was done for flax.
Nothing else in the repo references them: every test workflow filters
ONNX out with -k "not Onnx", run_nightly_onnx_tests is commented out,
and no doc or script names the images.

Add tests/others/test_docker_images.py, which asserts that the
directories under docker/ are exactly the images named in both jobs of
build_docker_images.yml, the PR-time ALLOWED_IMAGES array and the
nightly strategy.matrix.image-name list. It fails on both before this
change and passes after.

This does not remove the stale tags from Docker Hub, which needs
registry credentials.

Fixes #14325

Author: guptaishaan

docker/diffusers-onnxruntime-cpu/Dockerfile

@@ -0,0 +1,26 @@
+import os
+import re
+import unittest
+
+import yaml
+
+
+git_repo_path = os.path.abspath(os.path.dirname(os.path.dirname(os.path.dirname(__file__))))
+docker_path = os.path.join(git_repo_path, "docker")
+workflow_path = os.path.join(git_repo_path, ".github", "workflows", "build_docker_images.yml")
+
+
+class TestDockerImages(unittest.TestCase):
+    def setUp(self):
+        self.image_dirs = set(os.listdir(docker_path))
+        with open(workflow_path, "r") as f:
+            self.workflow = yaml.safe_load(f)
+
+    def test_images_are_built_on_pull_requests(self):
+        run = self.workflow["jobs"]["test-build-docker-images"]["steps"][-1]["run"]
+        allowed_images = re.search(r"ALLOWED_IMAGES=\(\n(.*?)\)", run, flags=re.DOTALL).group(1)
+        self.assertSetEqual(set(allowed_images.split()), self.image_dirs)
+
+    def test_images_are_pushed_on_schedule(self):
+        matrix = self.workflow["jobs"]["build-and-push-docker-images"]["strategy"]["matrix"]
+        self.assertSetEqual(set(matrix["image-name"]), self.image_dirs)